Valve assemblies

ABSTRACT

An electronically actuatable valve assembly including a valve member comprising a ferromagnetic member, and a magnetic circuit adapted to direct magnetic flux through the ferromagnetic member when the valve member is spaced apart from its valve seat, to hold the primary valve open.

FIELD OF THE INVENTION

The invention relates to the field of electronically actuated valveassemblies for regulating fluid communication between a manifold and aworking chamber of a fluid working machine.

BACKGROUND TO THE INVENTION

Fluid working machines include fluid-driven and/or fluid-drivingmachines, such as pumps, motors, and machines which can function aseither a pump or as a motor in different operating modes. Although theinvention will be illustrated with reference to applications in whichthe fluid is a liquid, such as a generally incompressible hydraulicliquid, the fluid could alternatively be a gas.

When a fluid working machine operates as a pump, a low pressure manifoldtypically acts as a net source of fluid and a high pressure manifoldtypically acts as a net sink for fluid. When a fluid working machineoperates as a motor, a high pressure manifold typically acts as a netsource of fluid and a low pressure manifold typically acts as a net sinkfor fluid. Within this description and the appended claims, the terms“high pressure manifold” and “low pressure manifold” refer to manifoldswith higher and lower pressures relative to each other. The pressuredifference between the high and low pressure manifolds, and the absolutevalues of the pressure in the high and low pressure manifolds willdepend on the application. For example, the pressure difference may behigher in the case of a pump which is optimised for a high power pumpingapplication than in the case of a pump which is optimised to preciselydetermine the net displacement of fluid, for example, a pump fordispensing a metered amount of fluid (e.g. a liquid fuel), which mayhave only a minimal pressure difference between high and low pressuremanifolds. A fluid working machine may have more than one low pressuremanifold.

Fluid working machines are known which comprise a plurality of workingchambers of cyclically varying volume, in which the displacement offluid through the working chambers is regulated by electronicallycontrollable valves, on a cycle by cycle basis and in phasedrelationship to cycles of working chamber volume, to determine the netthroughput of fluid through the machine. For example, EP 0 361 927disclosed a method of controlling the net throughput of fluid through amulti-chamber pump by opening and/or closing electronically controllablepoppet valves, in phased relationship to cycles of working chambervolume, to regulate fluid communication between individual workingchambers of the pump and a low pressure manifold. As a result,individual chambers are selectable by a controller, on a cycle by cyclebasis, to either displace a predetermined fixed volume of fluid or toundergo an idle cycle with no net displacement of fluid, therebyenabling the net throughput of the pump to be matched dynamically todemand. EP 0 494 236 developed this principle and includedelectronically controllable poppet valves which regulate fluidcommunication between individual working chambers and a high pressuremanifold, thereby facilitating the provision of a fluid working machinefunctioning as either a pump or a motor in alternative operating modes.EP 1 537 333 introduced the possibility of part cycles, allowingindividual cycles of individual working chambers to displace any of aplurality of different volumes of fluid to better match demand.

Fluid working machines of this type require rapidly opening and closingelectronically controllable valves capable of regulating the flow offluid into and out of a working chamber from the low pressure manifold,and in some embodiments, the high pressure manifold. Some aspects of theinvention aim to provide improved valve assemblies suitable forregulating the flow of fluid into and out of the working chamber offluid working machines of this type. However, the valve assemblies ofthe present invention are applicable to other types of fluid workingmachine.

A technical problem which can arise with valve assemblies includingelectronically actuatable face seating valves (such as poppet valves),for regulating the supply of fluid into the working chamber of a fluidworking machine, relates to the requirement to hold the face seatingvalve open whilst fluid is flowing through the valve. Bernoulli effects(kinetic energy related pressure drop) and surface friction arising fromthe flow of fluid past the face seating valve element (e.g. a poppethead) can exert a substantial force on the face seating valve element.Thus, it may be necessary to continue to supply a substantial amount ofpower to the electromagnet to keep the face seating valve open, or thiseffect may limit the maximum flow rate through the valve. Accordingly,the invention addresses the problem of holding open electronicallyactuatable face seating valves while fluid flows through the valves froma low or high pressure manifold to a working chamber of a fluid workingmachine, or in the reverse direction.

Some embodiments of the present invention also address the problem ofopening a face seating valve, such as a poppet valve, against a pressuredifferential, to regulate the supply of fluid from a high-pressuremanifold to a working chamber of a fluid working machine. This istechnically difficult because, in a face seating valve, the fluidpressure acts over the seating area to create a large closing force.Accordingly, it is difficult to provide a face seating valve forregulating the supply of fluid from a high-pressure manifold to aworking chamber of a fluid working machine which is capable of openingagainst a significant pressure differential and which also is alsocapable of opening quickly (ideally within a few milliseconds) whilstminimizing energy consumption.

GB 2,430,246 (Stein) discloses a valve assembly which is suitable forregulating the supply of fluid from a high-pressure manifold to aworking chamber of a fluid working machine. The valve assembly comprisesa primary valve, a secondary valve, an electromagnet and an armature(referred to as a moving pole). The primary valve comprising aface-seating primary valve member and a primary valve seat. Thesecondary valve is integral to the primary valve and includes asecondary valve member which is moveable between a sealing position andan open position in which a path is provided through the secondary valvefor fluid to flow between opposite sides of the primary valve member toreduce the pressure difference across the primary valve member. Thus,the secondary valve, which has a much smaller surface area than theprimary valve, can be opened even when there is a substantial pressuredifference across the primary valve member. The working chamber iseffectively a closed volume, and so fluid can flow through the secondaryvalve to equalise the pressure on either side of the primary valvemember, thereby facilitating the opening of the primary valve.

In the valve assembly disclosed in GB 2,430,246, the face seating valveis held open by a spring. In practice it is extremely difficult for thisspring to provide enough force to hold the valve open against theBernoulli and surface friction forces. Thus, some embodiments of thepresent invention aim to provide a valve assembly which is not onlycapable of remaining open in the presence of significant Bernoulli andsurface friction forces, but is also capable of opening quickly againsta significant pressure differential, whilst minimizing energyconsumption.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided avalve assembly for regulating the supply of fluid from a fluid manifoldto a working chamber of a fluid working machine, the valve comprising aprimary valve, an electromagnet and an armature, the primary valvecomprising a face-seating primary valve member and a primary valve seatand having an open position in which the primary valve member is spacedapart from the primary valve seat and a sealing position in which theprimary valve member is in sealing contact with the primary valve seat,wherein the armature is slidable along a path extending between a firstposition and a second position, and wherein, when the armature is in thefirst position, the primary valve member is biased towards the sealingposition, and wherein, when the armature is in the second position, theprimary valve member is biased towards the open position, characterisedin that the primary valve member comprises a ferromagnetic member andthe valve further comprises a magnetic circuit adapted to directmagnetic flux through the ferromagnetic member when the primary valve isopen to thereby hold the primary valve member in the open position.

Thus, when the primary valve is open and current is supplied to theelectromagnet, the primary valve member is located at a minimum ofmagnetic potential energy, resisting Bernoulli and surface frictionforces acting on the primary valve member due to the flow of fluid, pastthe primary valve member, through the valve seat. The valve assembly istherefore especially useful in fluid working machines where a relativelyhigh rate of fluid flow is anticipated in use. The valve assembly may,for example, be used to regulate the supply of fluid from a highpressure fluid manifold to a working chamber of a fluid working motor(such as a fluid working machine which functions solely as a motor or asa pump or motor in alternative operating modes). Nevertheless, the valveassembly can also be useful for regulating the supply of fluid from alow pressure manifold to a working chamber of a fluid working motor.

The primary valve member may comprise both the ferromagnetic member andone or more non-ferromagnetic valve member portions. However, theprimary valve member may consist entirely of the ferromagnetic member.

Because the primary valve member is biased towards the open positionwhen the armature is in the second position, preferably by one or moreelastic members, the primary valve member will be urged to a fully opencondition in which it provides less resistance to flow through the valvethan if it were biased to a closed position.

Preferably, the primary valve member is biased to the sealing positionwhen the armature is in the first position and the primary valve memberis biased to the closing position when the armature is in the secondposition by one or more elastic members. The armature is typicallymoveable relative to the primary valve member. Thus, by directingmagnetic flux through the ferromagnetic member when the primary valve isopen closure of the primary valve member due to forces acting on theprimary valve member from fluid flow, without concomitant movement ofthe armature, can be avoided.

Preferably, the primary valve member comprises a sealing region whichmakes sealing contact with the primary valve seat and the magneticcircuit is configured to direct flux through the sealing region when theprimary valve is open. Thus, the holding force is concentrated where itis most required.

The electromagnet is typically operable to attract the armature in useto thereby open the primary valve. Typically, the second position iscloser to the electromagnet than the first position. The magneticcircuit preferably directs magnetic flux from the said electromagnetthrough the ferromagnetic member when the primary valve is in the openposition to thereby hold the primary valve member while theelectromagnet is engaged. Thus, when current is supplied to theelectromagnet, it acts both to open the primary valve and to hold theprimary valve member in the open position. Alternatively, a secondelectromagnet could be provided within the magnetic circuit to provide amagnetic field to hold the primary valve member.

Preferably, the magnetic circuit is adapted to direct magnetic fluxthrough the ferromagnetic member both when the primary valve member isin the open position and when the primary valve member is in the sealingposition, wherein the magnetic circuit is adapted to direct a higherdensity of magnetic flux through the ferromagnetic member when theprimary valve member is in the open position. Thus, the magnetic circuitcan function both to increase the attractive force between theelectromagnet and the armature to open the primary valve, and to providea well of magnetic potential energy to hold the primary valve member inthe open position.

Preferably, the magnetic circuit comprises first and second magneticcircuit portions which are arranged to conduct magnetic flux inparallel, wherein the first magnetic circuit portion is configured toconduct magnetic flux through the armature, at least when the primaryvalve is in the sealing position and the armature is in the firstposition, and the second magnetic circuit portion is configured toconduct magnetic flux through the ferromagnetic member, at least whenthe primary valve is in the open position and the armature is in thesecond position. Preferably, the first magnetic circuit portion isconfigured to conduct magnetic flux through the armature both when thearmature is in the first position and when the armature is in the secondposition. The second magnetic circuit portion may be configured toconduct magnetic flux through the ferromagnetic member both when theprimary valve member is in the open position and when the primary valvemember is in the sealing position.

The first magnetic circuit portion preferably comprises a flux bridgearranged to directed flux through the armature, at least when thearmature is in the first position and typically also when the armatureis in the second position. Typically, the armature is adapted to slidealong an axis between the first and second positions. The flux bridgemay, for example, comprise a plurality of radially inwardly extendingmagnetic circuit members arranged substantially normally to the saidaxis to direct magnetic flux through the armature. The armature maycomprise a peripheral flange of magnetically permeable material,proximate to the flux bridge, through which a magnetic circuit can beformed when the armature is in the first position, the second positionor between the first and second positions.

Typically, the magnetic circuit comprises a ferromagnetic body portionand the armature is spaced apart from the ferromagnetic body portion inthe first position and contacts the ferromagnetic body portion in thesecond position.

Preferably, the second magnetic circuit portion is arranged so that whenthe primary valve member is in the open position, a continuous body offerromagnetic material extends directly between the ferromagnetic memberand the ferromagnetic body portion. It may be that, in the openposition, the ferromagnetic member contacts the armature whilst thearmature contacts the ferromagnetic body portion. It may be that, in theopen position, the ferromagnetic member directly contacts the bodyportion. The ferromagnetic member may comprise a protruberance extendingtowards the body portion and contacting the body portion when theprimary valve member is in the open position. The body portion maycomprise a protruberance extending towards the ferromagnetic member andcontacting the primary valve member when the primary valve member is inthe open position.

Preferably, the magnetic circuit is formed and arranged so that theratio of the reluctance of the first magnetic circuit portion to thereluctance of the second magnetic circuit portion is higher when thearmature is in the second position and the primary valve member is inthe open position than when the armature is in the first position andthe primary valve member is in the sealing position. Preferably, thereluctance of the interface between the flux bridge and the armature(which may be a sliding contact or a small gap) is higher when thearmature is in the second position. For example, the flux bridge and thearmature may overlap with a greater overlap surface area when thearmature is in the first position than when the armature is in thesecond position. This increases the proportion of the magnetic fluxdirected through the primary valve member when the primary valve memberis open, holding the primary valve member open. In some embodiments,this arrangement facilitates the opening of or holding open of theprimary valve member by providing an attractive force between theferromagnetic member and the armature or body portion.

Preferably, the first magnetic circuit portion and armature areconfigured so that the reluctance of the first magnetic circuit portionis greater when the armature is in the second position than in the firstposition, to thereby increase the magnetic flux directed through theferromagnetic member when the armature is in the second position and theprimary valve member is in the open position.

The second magnetic circuit portion may be configured to conductmagnetic flux through the ferromagnetic member both when the primaryvalve member is in the open position and when the primary valve memberis in the sealing position. The ferromagnetic member may contact thearmature when the armature is in the first position and the primaryvalve member is in the sealing position. In this arrangement, flux willbe directed preferentially through the flux bridge and armature ratherthan through the ferromagnetic member, facilitating movement of thearmature from the first position without immediate movement of theprimary valve member.

Typically, the primary valve member and armature are coaxial.Preferably, the valve assembly comprises an elastic member arranged tobias the primary valve member away from the primary valve seat and anelastic member arranged to bias the armature to contact the primaryvalve member such that the resultant forces bias the primary valvetowards the sealing position.

The adjective “primary” in “primary valve”, “primary valve member” andrelated terms, is used as a label for clarity and is not intended toimply that there is a secondary valve. Nevertheless, in someembodiments, the valve assembly may further comprise a secondary valvecoupled to the armature, the secondary valve comprising a secondaryvalve member moveable between a sealing position and an open position,wherein when the armature is in the first position the secondary valveis biased to the sealing position and, when the armature is in thesecond position, the secondary valve is biased towards the openposition.

In this case, the coupling between the armature and the secondary valveis preferably configured to enable the armature to move from the firstposition towards the second position without a corresponding movement ofthe secondary valve member, but to exert a force through the couplingbetween the armature and the secondary valve member to cause thesecondary valve member to move and to thereby open the secondary valve,while the armature is at a location between the first position and thesecond position along the said path. The secondary valve member isarranged to provide a path for fluid to flow between opposite sides ofthe primary valve member in the open position so that, in use, whenthere is a pressure differential across the primary valve member whichapplies a force maintaining the primary valve member in sealing contactwith the primary valve seat, opening of the secondary valve memberenables pressure to be equilibrated on either side of the primary valvemember to facilitate the opening of the primary valve member.

Thus, the valve assembly may be a valve assembly for regulating thesupply of fluid from a high-pressure manifold to a working chamber of afluid working machine, the valve assembly further comprising a secondaryvalve, the secondary valve comprising a secondary valve member moveablebetween a sealing position and an open position in which a path isprovided through the secondary valve for fluid to flow between oppositesides of the primary valve member to reduce the pressure differenceacross the primary valve member, wherein the armature is coupled to thesecondary valve member and the second position is closer to theelectromagnet than the first position, wherein, in the first position,the secondary valve is biased towards the sealing position and, in thesecond position, the secondary valve is biased towards the openposition, wherein the coupling between the armature and the secondaryvalve member is configured to enable the armature to move from the firstposition towards the second position without a corresponding movement ofthe secondary valve member, but to exert a force through the couplingbetween the armature and the secondary valve member to cause thesecondary valve member to move and to thereby open the secondary valve,while the armature is at a location between the first position and thesecond position along the said path.

In contrast to the valve arrangement disclosed in GB 2,430,246, thearmature is part way along the path from the first position to thesecond position, and thereby closer to the electromagnet, when the forcethrough the coupling between the armature and the secondary valve membercauses the secondary valve member to move and to thereby open thesecondary valve. As the armature travels towards the electromagnet, theattractive force acting on the armature due to the electromagnet shouldincrease. Accordingly, the force which can be applied to the secondaryvalve member by the armature, through the coupling between the armatureand the secondary valve member, is greater than would be the case in avalve such as that disclosed in GB 2,430,246 where the armature is atits furthest point from the electromagnet when the secondary valvemember begins to move. This may enable the primary valve to open morequickly, more reliably, at higher pressure, or with less powerconsumption than if the armature was fixedly coupled to the secondaryvalve member. Thus a valve assembly is provided in which a relativelyhigh opening force can be exerted on the secondary valve member and arelatively high force can be provided to hold the primary valve memberin the open position.

Preferably, the secondary valve member is a face-seating valve and thesecondary valve further comprises a secondary valve seat for sealablecooperation with the secondary valve member. The secondary valve istypically oriented in the same direction as the primary valve, so that,when the valve assembly is employed in a fluid-working machine such thatthere is fluid pressure differential which applies a force holding theprimary valve member in sealing contact with the primary valve seat, aforce in the same sense holds the secondary valve member in sealingcontact with the secondary valve seat. The invention is of specialbenefit where the secondary valve is a face-seating valve as asubstantial force can be required to open a face-seating secondary valveagainst a pressure differential. Preferably, the cross-section of thesecondary valve seat is less than 10% of, and more preferably less than5% of the cross-section of the primary valve seat, so that the forceholding the secondary valve closed due to the pressure differencebetween the inlet and the outlet is substantially less than thecorresponding force holding the primary valve closed.

Preferably, the secondary valve extends through the primary valveelement to enable fluid to flow through the primary valve element whenthe secondary valve is in the open position to reduce the pressuredifferential between opposite sides of the primary valve member. Thus,the secondary valve seat may be integral to the primary valve element.The primary valve element and the secondary valve element may be coaxialand preferably move along coaxial paths in use.

Preferably, the coupling between the armature and the secondary valvemember comprises an elastic member (the secondary elastic member) whichis operable to store elastic energy as the armature travels along thepath from the first position towards the second position. This providesa mechanism to enable the armature to move from the first positionwithout the secondary valve member beginning to move, and enables theforce acting on the secondary valve member to increase until it exceedsthe force required to open the secondary valve member against a pressuredifferential in use.

Preferably, the force exerted on the secondary valve member through thecoupling increases monotonically as the armature moves from the firstposition towards the second position, at least until the secondary valvemember begins to move. The armature may move in a straight lineextending from the first position towards the second position, directlytowards the electromagnet.

The coupling between the armature and secondary valve member maycomprise (or consist of) a distance limiting mechanism which isengageable to limit the maximum distance between the armature and thesealing part of the secondary valve member and to thereby couplemovement of the armature to movement of the secondary valve along aportion of the path between the first position and the second position.In this case, the armature can begin to move from the first positiontowards the second position without movement of the secondary valvemember, however, the armature will reach a position between the firstposition and the second position where the distance limiting mechanismengages and further motion of the armature must be coupled toconcomitant movement of the secondary valve member. The coupling betweenthe armature and the second valve member may comprise either or both ofthe said distance limiting mechanism and the said secondary elasticmember.

Preferably, the valve assembly comprises a substantially rigid stemwhich extends through an aperture in the armature which is fixedlycoupled to the secondary valve member on a first side of the armaturelocated towards the working chamber in use and coupled to the armatureby an elastic member on the opposite second side of the armature, suchthat the armature extends around and slides along the substantiallyrigid stem in use as the armature travels along the path from the firstposition. Preferably, the substantially rigid stem further comprises aformation on the second side of the armature which engages with thearmature at the said location between the first position and the secondposition so that the armature drags open the secondary valve at the saidlocation between the first position and the second position. In thiscase, the said formation and the surface of the armature which engageswith the said formation can together form the distance limitingmechanism.

The armature and primary valve member may be coupled by a distancelimiting mechanism which is engageable to limit the maximum distancebetween the armature and the primary valve member to thereby couplemovement of the armature to movement of the primary valve member along aportion of the path between the first position and the second position.For example, the armature may comprise a primary valve member engagingformation to engage with and open the primary valve when the armature isbetween the first position and the second position.

The armature may be operable to engage first with the secondary valveand then with the primary valve member as the armature moves from thefirst position to the second position during opening of the valve topull the secondary valve open before the primary valve member begins tomove and then to unseat the primary valve member from the primary valveseat.

Typically, at least one elastic member, including the said secondaryelastic member, together exert a biasing force on the armature to biasthe armature towards the first position. Preferably also, the saidbiasing force increases as the armature moves from the first positiontowards the second position, wherein the increase in biasing force withdistance is less than the increase in the force exertable by theelectromagnet on the armature in use, due to the reduction in thedistance between the electromagnet and the armature, at least where thearmature is located between the first position and the position wherethe secondary valve member begins to move.

The valve assembly may comprise a primary elastic member which biasesthe primary valve member towards sealing contact with the primary valveseat. Preferably, the primary elastic member biases the armature towardsthe first position and the armature engages with the primary valvemember when the armature is in the first position so that, in the firstposition, the primary elastic member biases the armature into contactwith the primary valve member and thereby biases the primary valvemember into sealing contact with the primary valve seat.

Valve assemblies in which an elastic member (e.g. the said primaryelastic member) acts directly on the armature (e.g. referenced to thevalve body and to the armature) to urge the armature to engage with theprimary valve member and urge the primary valve member towards sealingcontact with the primary valve seat are advantageous because they beginto close immediately that the armature is no longer held in the secondposition by the electromagnet. Thus, they can be closed rapidly. Theycan also be energy efficient as energy stored in the elastic memberwhich acts directly on the armature during opening is used in closing.

Where, the valve assembly comprises a secondary elastic member which isreferenced to the armature and the secondary valve member (for example,referenced to a substantially rigid stem which is integral with orattached to the secondary valve member and which extends through anaperture in the armature), and a primary elastic member referenced tothe valve body and to the armature, the primary and secondary elasticmembers may be concentric springs, with the primary elastic memberextending around the secondary elastic member. This arrangementfacilitates the provision of a compact valve assembly and enables theaxial extent of the valve to be minimised.

Preferably, the secondary valve is biased into the closed position, forexample by an elastic member. Thus, where the secondary valve is a faceseating valve, the secondary valve member may be biased into sealingcontact with the secondary valve seat, for example by an elastic member.

An elastic member is preferably provided which biases the primary valvemember away from the primary valve seat. The said elastic member may,for example, be referenced from the armature or the body of the valveassembly. Preferably, the said elastic member is arranged so that theforce exerted by the said elastic member increases once the secondaryvalve is open. However, the said elastic member should exert a lowerbiasing force than the primary elastic member when the armature is inthe first position so that the net biasing force on the primary valvemember, when the armature is in the first position, biases the primaryvalve member into sealing contact with the primary valve seat.

A tertiary elastic member may be provided which has a first end which isfixed to the secondary valve member and a second end which is fixed tothe primary valve member. Thus, the tertiary elastic member may functionboth to bias the secondary valve into a closed position and to bias theprimary valve member away from the primary valve seat. The tertiaryelastic member should exert a lower biasing force than the primaryelastic member when the armature is in the first position so that thenet biasing force on the primary valve member, when the armature is inthe first position, and the secondary valve is closed biases the primaryvalve member into sealing contact with the primary valve seat. Thetertiary elastic member is typically compressed, or further compressed,when the secondary valve first opens and therefore provides anadditional force biasing the primary valve member away from the primaryvalve seat.

Preferably, an elastic member which is referenced to the secondary valvemember, for example the said tertiary elastic member, may be located ina recess within the primary valve member, for example, within a borewhich defines part of the said path through the secondary valve. Thus,the said elastic member may be shielded by the primary valve member fromhigh fluid flow. The elastic member may be located entirely within arecess in the primary valve member when the armature is in contact withthe primary valve member. Preferably, the said elastic member isoperable to urge the primary valve member to unseat from the primaryvalve seat at least when the secondary valve is in the open position.

The armature may be coupled to the primary valve through the couplingbetween the armature and the secondary valve, for example, through thesecondary elastic member and the tertiary elastic member.

It may be that the primary valve is biased open predominantly or solelyby an elastic member referenced between the primary valve member and thesecondary valve member. If, instead, the primary valve was biased openby an elastic member referenced between the valve body and the primaryvalve member, this would provide a force which would require to beovercome during closing, slowing closure of the valve and increasing theforce required from any closing elastic member and thereby increasingthe energy consumed to compress said closing elastic member on opening.

Preferably, the location of the armature where the coupling between thearmature and the secondary valve member is operable to cause thesecondary valve member to move is more than 50%, or more preferably morethan 75%, of the distance from the first position to the secondposition.

Preferably, once the secondary valve member begins to move, it moves bya greater distance than the remaining travel of the armature along thepath from the first position to the second position.

Typically, when the armature is in the second position, the secondaryvalve is or may be closed, particularly when the primary valve is open.The secondary valve should typically open to enable the primary valve toopen although it may not be necessary for the secondary valve to remainopen once the primary valve has opened.

Preferably, the interior of the valve assembly is configured to minimiserestrictions on the movement of the armature from the first position dueto a requirement to displace hydraulic fluid. Preferably, movement ofthe armature is not significantly restricted by the flow of hydraulicfluid through a throttle (e.g. an aperture sized to cause a significantpressure difference to develop across the aperture in use). This reducesforces restricting the armature from moving, slowing opening and/orclosing of the valve.

The secondary valve member typically moves into a secondary valve memberreceiving volume when the secondary valve moves from the sealingposition into the open position. The secondary valve member receivingvolume is typically filled with hydraulic fluid in use which isdisplaced by the movement of the secondary valve member. In contrast topressure balancing valves, the movement of the armature from the firstposition typically has no effect on or increases the pressure ofhydraulic fluid in the secondary valve member receiving volume. This isadvantageous because a reliance on movement of the armature to lower thepressure within the secondary valve member receiving volume can slow theopening of the valve and in particular slow its closing. Preferably theflow of hydraulic fluid into and out of the secondary valve memberreceiving volume is not throttled. Thus, the valve may close quicklywithout throttled flow slowing the flow of hydraulic fluid into thesecondary valve member receiving volume.

The invention extends in a second aspect to a fluid working machinecomprising a working chamber of cyclically varying volume, a highpressure manifold and a low pressure manifold, and a valve assemblyaccording to the first aspect of the invention which regulates thesupply of fluid from the high pressure manifold or the low pressuremanifold to the working chamber.

When the secondary valve opens, fluid can flow from the respectivemanifold through the secondary valve to the working chamber. As aworking chamber is a closed chamber (albeit one of cyclically varyingvolume) the pressure within the working chamber can equilibrate withpressure within the respective manifold, to reduce the pressuredifference across the primary valve member and enable the primary valveto open. Preferably, the pressure difference across the primary valvemember is reduced primarily by the pressure within the working chamberchanging towards equilibrium with the pressure in the respectivemanifold as a result of fluid flowing through the secondary valve, afterthe secondary valve member is moved to the open position, before theprimary valve member unseats from the primary valve seat. Preferably,the pressure difference across the primary valve member is not reducedprimarily by providing a chamber within the valve assembly, incommunication with the primary valve member, the pressure within whichis reduced below the pressure in the respective manifold to enable theprimary valve member to open.

The fluid working machine may further comprise a controller which isoperable to actively control the said valve assembly, and optionally oneor more other valves, in phased relation to cycles of working chambervolume, to determine the net displacement of the fluid by the or eachworking chamber on a cycle by cycle basis, to thereby determine the timeaveraged net displacement of fluid by the working machine or one of moregroups of said working chambers.

Preferably, the pressure difference between the high pressure manifoldand the low pressure manifold and the throughput of fluid through thevalve assembly, are such that the fluid working machine would notfunction correctly if the primary valve member does not comprise aferromagnetic member because, in at least some operating conditions, theprimary valve would close too soon due to Bernoulli effects and/orsurface friction due to fluid flowing past the primary valve head.

The fluid working motor may function only as a motor, or only as a pump.Alternatively, the fluid working motor may function as a motor or a pumpin alternative operating modes.

DESCRIPTION OF THE DRAWINGS

An example embodiment of the present invention will now be illustratedwith reference to the following Figures in which:

FIG. 1A is a part cross-section through a symmetric valve assembly whilethe valve is closed and before current is passed through theelectromagnet;

FIG. 1B is a part cross-section through the valve assembly of FIG. 1Aafter the armature has moved from the first position, towards the secondposition responsive to the magnetic field generated by current beingpassed through the electromagnet;

FIG. 1C is a part cross-section through the valve assembly of FIG. 1Aafter the secondary valve member has moved from its valve seat to openthe secondary valve;

FIG. 1D is a part cross-section through the valve assembly of FIG. 1Aafter the primary valve member has moved from the primary valve seat toopen the primary valve and the armature has reached the second position;

FIG. 2 is a graph of the variation with the location of the armature offorces acting on the rigid stem and secondary valve member within thevalve assembly of FIG. 1A (forces acting in an inwards direction havepositive values); and

FIG. 3 is a schematic diagram of a fluid working machine incorporatingthe valve assembly of FIG. 1A.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

With reference to FIGS. 1A through 1D, a valve assembly 1 according tothe present invention has an annular valve housing 2, made from amagnetically permeable material, which encompasses a body portion 4,also made from a magnetically permeable material. A ring of highreluctance material 30 separates the valve housing from the bodyportion. An electromagnet 6 is formed around the body portion, withinthe valve housing. An annular poppet cage 8 extends from the valvehousing and encompasses a primary poppet valve head 10, which functionsas the primary valve member. The annular poppet cage is made from amagnetically permeable material and the primary poppet valve head ismade from a ferromagnetic material, such as steel, and so functions asthe ferromagnetic member. A valve seat 12 (functioning as the primaryvalve seat) is formed by a bevelled transition extending around theinterior of the poppet cage. In a closed position, the primary poppetvalve head mates with the primary valve seat to form a seal. Thearmature and primary poppet valve are configured so that the outwardsurface 62 of the armature can contact the inward surface 64 of theprimary poppet valve head in order to apply force between the two parts,for example, when the valve is closed before a current is passed throughthe electromagnet.

The primary poppet valve head includes an aperture 14 located on thecentral axis of the primary poppet valve head. The aperture extends to afurther bevelled transition 16, located within a valve head chamber 18,within the body of the primary poppet valve head, which also functionsas a valve seat (the secondary valve seat), against which a secondaryvalve member 20 is biased, to form a seal, when the valve assembly isfully closed. The aperture communicates with an interior chamber 22,within the annular poppet cage, by way of the valve head chamber, andone or more of fluid passages 24. The fluid passages have sufficientcross-sectional area to avoid significantly restricting fluid flow.Thus, when the secondary valve member is not in sealing contact with thesecondary valve seat, a path is provided for fluid to flow between theoutlet 26 of the valve, and the interior chamber 22. However, this pathis closed off when the secondary valve member is in sealing contact withthe secondary valve seat.

The interior chamber is in fluid communication with one or more radialpassages 28 which extend through the poppet cage, and function as inletsto the valve assembly. The radial passages extend into the interiorchamber at a location which is intermediate the location of theperiphery of the primary poppet valve head when the primary poppet valveis open, and the location of the periphery of the primary poppet valvehead when the primary poppet valve head is in sealing contact with theprimary poppet valve seat. Accordingly, a path is provided for fluid toflow directly from the inlets to the outlet, when the primary poppetvalve is open, irrespective of whether the secondary valve is open.However, no path is provided for fluid to flow directly from the inletsto the outlet, around the periphery of the primary poppet valve head,when the primary poppet valve is closed.

A magnetic circuit is formed in part by a ring of high reluctancematerial 30, which is located around the periphery of the body portion.The magnetic circuit also includes a flux bridge 32, which extendsradially inwards from the valve housing and contacts an armature 34,which is slidable from a first position, illustrated in FIG. 1A, to asecond position, illustrated in FIG. 1B. The armature has a peripheralflange 36, which is thicker than the central portion of the armature,and which is in sliding contact with the flux bridge, and configured soas to remain in contact with (or, alternatively, remain very close to),the flux bridge whilst the armature is at any location between the firstposition and the second position. The flux bridge includes one or morethrough-bores 38, through which fluid can flow to enable the armature tomove between the first and second positions. The annular valve housing,body portion and flux bridge together form a first magnetic circuitportion. A second magnetic circuit portion is formed by the annularvalve housing, body portion and annular poppet cage, which is also madefrom a magnetically permeable material, such as steel, and in contactwith the flux bridge.

The armature has a central aperture therethrough 40. A rigid stem 42extends through the central aperture of the armature, and the chamberwithin the primary poppet valve head. The rigid stem has a first endwhich forms the secondary valve member 20, and an opposite second end44, located within a recess 46, within the valve body portion.

The valve assembly comprises three springs. The main spring 48(functioning as the primary elastic member) extends around the rigidstem from a transition 50 within the body portion recess to an inwardsurface 52 of the armature and is in compression throughout operation. Acharge spring 54 (functioning as the secondary elastic member) extendsfrom the inward surface of the armature, around the rigid stem, and islocated on a peripheral flange 56 at the second end of the rigid stem.The peripheral flange also has an outward surface 66 against which theinward surface of the armature can react, so that the inward surface ofthe armature and the outward surface of the peripheral flange therebyform a distance limiting mechanism. A pilot spring 58 (functioning asthe tertiary elastic member) extends between a radially outwardlyextending peripheral flange 60 located towards the first end of therigid stem, and a radially inwardly extending flange around the interiorof the chamber within the primary poppet valve head. The pilot spring isrelatively relaxed when the valve assembly is in the fully closed stateillustrated in FIG. 1A but in compression when the secondary valve hasopened but the primary valve has not opened, illustrated in FIG. 1C.

The primary valve member, the secondary valve member, the rigid stem,and each of the main, charge and pilot springs are coaxial. The mainspring is concentric with and extends around the charge spring.

In an example application, the valve assembly is located within a fluidworking machine, with the inlet connected to a high pressure manifold,and the outlet attached to a working chamber of cyclically varyingvolume. The electromagnet is connected to a current source which isswitchable under the control of a controller to enable current to besupplied to the electromagnet when required.

When no current is supplied to the electromagnet, the valve adopts theclosed position illustrated in FIG. 1A. The main spring provides abiasing force in an outwards direction and so the armature is biased inan outwards direction, pressing the primary poppet valve head intosealing contact with the primary valve seat through the contact acrossoutward surface 62 and inward surface 64. The charge spring is relaxedand so exerts only a small force on the rigid stem in an inwardsdirection (i.e. towards the top of FIG. 1A). The pilot spring exerts anopposite and typically higher force on the rigid stem in an outwardsdirection. For example, the charge spring may have a preload of 10N andthe pilot spring may have a preload of 15N. Thus, the net force on therigid stem due to the preload within the charge spring and the pilotspring biases the rigid stem, and therefore the secondary valve member,outwards, into sealing contact with the secondary valve seat. Theprimary valve and secondary valve are also retained in the closedpositions by the pressure differential between the internal chamber ofthe valve assembly, and the outlet. Accordingly, in the closed positionillustrated in FIG. 1A, the valve assembly is closed and there is nopath for fluid to flow from the high pressure valve, through the inlet,to the outlet and into the working chamber.

When current is supplied to the electromagnet, a magnetic circuit isformed, guiding flux through the armature. The electromagnet exerts anattractive force on the armature and the current through theelectromagnet is selected so that the force acting on the armature issufficient to move the armature from the first position to the secondposition. In a typical application, the attractive force would beinsufficient to move the armature if the armature was fixedly coupled tothe secondary valve member. However, according to the invention theelastic coupling allows the armature to move initially without movementof the secondary valve member. It is therefore possible to avoidunnecessary power expenditure by using a lower initial attractive forcethan was previously necessary. As the armature moves from the firstposition to the second position, the gap between the armature and thebody portion decreases and the force on the armature increases.

FIG. 2 is a graph of the variation in relevant forces whilst thearmature travels along a path from a first position 100 to a secondposition 102 which is closer to the electromagnet than the firstposition. The path extends directly from the first position to thesecond position, straight towards the central axis of the toroidalelectromagnet. The force from the pilot spring 104 (which acts in aninwards direction on the primary poppet valve head and an outwards(negative in the graph) direction on the rigid stem and integralsecondary valve member) is constant as the armature begins to move, asthe primary valve and secondary valve remain closed, held in place bythe biasing forces and the pressure differential. The force from thecharge spring 106 (which acts in an outwards direction on the armatureand in an inwards direction (positive in the graph) on the rigid stemand therefore the secondary valve member) increases monotonically as thearmature begins to move, without movement of the rigid stem, due toshortening of the charge spring. The force from the main spring 108(which acts in an outwards direction on the armature), also increasesmonotonically as the armature moves from the first position to thesecond position. The pressure differential between the inlet and outletof the valve assembly remains constant while the secondary valve remainsclosed and a constant force 110 in an outwards direction acts on thesecondary valve member (and thereby the rigid stem, negative in thegraph) as a result.

The total resultant force on the secondary valve is shown as line 112and it can be seen that at the first position 100 this is initiallysignificantly more than the total force 114 which can be exerted by theelectromagnet in the opposite (inwards, opening) direction. However, thetotal force 114 is larger than the net force on the armature from theopening force 106 coming from the charge spring 54 and the pilot force104 from the pilot spring 58, allowing the armature to move away fromthe first position 100. As the armature moves from the first positiontowards the open position the opening force on the armature exerted bythe charge spring increases linearly but the total force which theelectromagnet exerts on the armature increases with second ordercomponents and exceeds the resultant force required to open thesecondary valve member when the armature reaches an opening position 101between the first and second positions. The location of the openingposition will vary depending on the pressure differential between theinlet and the outlet. The configuration of the valve assembly when thearmature reaches the opening position is illustrated in FIG. 1B.

In the example of FIG. 2 the net force generated on the rigid stem fromthe combination of the charge and pilot springs is not large enough initself to move the rigid stem against the fluid forces 110 which resultfrom the pressure difference across the primary valve member. It ispossible that at low pressure differentials the net force would be highenough and the rigid stem would move inwards, moving the secondary valvemember out of sealing contact with the secondary valve seat, and openingthe secondary valve, as illustrated in FIG. 1C. However, in applicationswith sufficiently high pressure differentials, the inward surface 52 ofthe armature contacts the outward surface 66 of the peripheral flange 26at the opening point 101. Thus, the maximum distance between thearmature and the secondary valve member is limited and there is a suddenincrease 116 in the opening force 106 which is applied to the rigid stemby the armature. The forces acting on the rigid stem are now sufficientto move the rigid stem inwards, moving the secondary valve member out ofsealing contact with the secondary valve seat, and opening the secondaryvalve, as illustrated in FIG. 1C. It can be seen that the force appliedby the charge spring in combination with the contacting inward andoutward surfaces of the armature and the peripheral flange respectivelyis much higher than the armature would be able to supply in the openposition 100. However, because the armature has moved almost to theclosed position in the example illustrated in FIG. 2, a much higheropening force is available than would otherwise be the case.

Once the secondary valve opens, it provides a path of relatively smallcross-section for fluid to flow from the internal chamber of the valveassembly, through the secondary valve seat and the central aperture inthe primary poppet valve head, to the outlet. The outlet is connected toa working chamber which is effectively a closed volume, as the openingprocess happens so quickly that any change in working chamber volume isnegligible. Accordingly, as high-pressure fluid is supplied to theworking chamber through the secondary valve, the pressure at the inletand the outlet begins to equilibrate due to an increase in pressure atthe outlet. The total force required to move the rigid stem begins todrop 118, as the pressure differential drops. The force within thecharge spring begins to drop 116, as the rigid stem begins to moverelative to the primary poppet valve head, and the force within thepilot spring begins to increase 120, as the rigid stem moves relative tothe primary poppet valve head, thereby reducing the length of the pilotspring. The rigid stem will settle in a position where the force fromthe pilot spring and the force from the charge spring are equal.

Once the secondary valve is open, the pressure differential across theprimary poppet valve member drops rapidly, and the primary valve membercan be readily unseated from the primary valve seat and displaced to theposition illustrated in FIG. 1D by virtue of the forces transmittedthrough the rigid stem and pilot spring. Fluid can therefore flow freelyfrom the inlet to the outlet. The secondary valve will typically remainopen when the valve assembly is in the open position, however, this isnot essential. The primary valve will be biased open as the net forceexerted by the pilot spring now exceeds the net force exerted by thecharge spring, so long as the armature is held in the second position,thereby removing the force of the main spring that would otherwise actto close the primary valve. The primary valve will close again, due tothe action of the main spring, and the secondary valve will close again,due to the higher preload force of the pilot spring over the chargespring, when the current flowing through the electromagnet is cut off.As the main spring acts directly on the armature which bears directlyagainst the primary valve member, the primary poppet valve head startsto move towards the primary valve seat rapidly once the current flowingthrough the electromagnetic is cut off. Because it is referenced to theconcomitantly-moving secondary valve, the pilot spring which opens theprimary valve does not appreciably compress during closure, ensuringthat the full force of the main spring is available to accelerate theclosing.

In the illustrated valve arrangement, a magnetic circuit is formedcomprising a first magnetic circuit portion, which extends through theflux bridge and the armature, and a second magnetic circuit portionwhich extends through the poppet cage and the primary poppet valve head.The first and second magnetic circuit portions are in parallel.

When the valve assembly is in the position illustrated in FIG. 1A andcurrent is first supplied to the electromagnet, magnetic flux isconducted predominantly through the annular valve housing, flux bridgeand armature, and across the gap between the armature and body portion.The flux density through the poppet cage and primary poppet valve headis relatively low as the reluctance of the first magnetic circuitportion is substantially lower than the reluctance of the secondmagnetic circuit portion.

The armature is attracted to the body portion and begins to move towardsthe body portion. The peripheral flange around the armature is arrangedso that the flux bridge nearly contacts the armature across the entirecross-sectional area of the flux bridge not only when the armature is inthe first position but through the first part of the movement of thearmature. The armature moves away from the primary poppet valve headwhich cannot initially move and the reluctance of the second magneticcircuit portion increases still further.

Once the armature reaches the second position, after the secondary valvehas started to open, the armature contacts the body portion and remainsheld in place against the body portion while a current continues to flowthrough the electromagnet. However, the peripheral flange is arranged sothat, in the second position, the overlap between the peripheral flangeand the flux bridge is significantly less than the entirecross-sectional area of the flux bridge. This increases the reluctanceof the junction between the flux bridge and the armature.

After a short period of time, the secondary valve member contacts thearmature which itself remains in contact with the body portion. Whilecurrent continues to be supplied through the electromagnet, a magneticcircuit is completed through the electromagnet, the poppet cage, theprimary poppet valve head and the armature. Thus, in the open position,the primary poppet valve head is magnetically attracted to the armature,and is subject to a force, which resists Bernoulli forces which act onthe primary poppet valve head, caused by the rapid flow of fluid, fromthe inlet, past the periphery of the primary poppet valve head to theoutlet and then to a working chamber, down a pressure gradient. Thus,the primary poppet valve head is held open by virtue of the magneticcircuit.

This mechanism is facilitated by the increased reluctance of thejunction between the flux bridge and the armature, which directs fluxthrough the poppet valve cage and the primary poppet valve head(functioning as the second magnetic circuit portion). Even though theoverall reluctance of the path through the flux bridge and armature tothe body portion may be lower when the armature is in the secondposition than the first position because the armature is in directcontact with the body portion, the ratio of the reluctance of the paththrough the flux bridge and armature to the body portion to thereluctance of the path through the primary valve poppet head is higherwhen the armature is in the second position and the primary valve poppethead is held open. Thus, a higher proportion of magnetic flux aredirected through the primary valve poppet head than would otherwise bethe case, increasing the available holding force.

Once the current to the electromagnet is switched off, the magneticfield decays and the primary poppet valve head moves in an outwardsdirection to sealingly contact the primary valve seat.

In alternative embodiments, the primary valve poppet head may directlycontact the body portion in the open position. The primary valve poppethead or the body portion may comprise a protruberance to facilitate thisdirect contact.

In alternative embodiments, corresponding magnetic circuitry could beemployed to hold open the valve member of valve assemblies which do notinclude secondary valves.

Although the primary valve poppet head is made entirely fromferromagnetic material in this example embodiment, one skilled in theart will appreciate that the primary valve member may comprise both aferromagnetic region and a non-ferromagnetic region.

The valve arrangement disclosed herein has significant advantages overthe valve arrangement of GB 2,430,246. The armature is able to moveclose to the electromagnet before the secondary valve opens so as to beable to provide the maximum opening force on the secondary valve, butthe secondary valve is able to have the same travel as before as once itstarts moving, and is therefore subject to a reduced force from fluidpressure as the pressure in the working chamber equalises, it is fullyopened by the action of the compressed secondary spring. Also, theelectromagnet is able to apply attractive magnetic force to the primaryvalve to hold it open, directly applying it rather than only through theaction of a spring which is extended and therefore only able to providea weak holding force. These advantages means that a valve can beprovided which consumes less power and which can open against greaterpressure differentials.

FIG. 3 is a schematic diagram of a fluid working machine, showngenerally as 200, incorporating the illustrated valve assembly 202 as ahigh pressure valve, which regulates the flow of hydraulic fluid betweena high pressure manifold 204 and a working chamber 206. The workingchamber is defined by the interior of a cylinder 208 and a piston 210which is mechanically linked to the rotation of a crankshaft 212 by asuitable mechanical linkage 214, and which reciprocates within thecylinder to cyclically vary the volume of the working chamber. A lowpressure valve 216 regulates the flow of hydraulic fluid between a lowpressure manifold 218 and the working chamber. The example fluid workingmachine includes a plurality of working chambers and mechanically linkedto the rotation of the same crankshaft, with appropriate phasedifferences. A shaft position and speed sensor 220 determines theinstantaneous angular position and speed of rotation of the shaft, andtransmits shaft position and speed signals to a controller 222, whichenables a controller to determine instantaneous phase of the cycles ofeach individual working chamber. The controller is typically amicroprocessor or microcontroller which executes a stored program inuse. The low pressure valve is electronically actuatable, and theopening and/or the closing of the high and low pressure valves is underthe active control of the controller.

The example fluid working machine is operable to function as either apump or a motor in alternative operating modes. When operating as apump, low pressure fluid is received from the low pressure manifold, andoutput through the high pressure valve to the high pressure manifold.Shaft power is therefore converted into fluid power. When operating as apump, high pressure fluid is received from the high pressure manifold,and output through the low pressure valve to the low pressure manifold.Fluid power is therefore converted into shaft power.

The controller regulates the opening and/or closing of the low and highpressure valves to determine the displacement of fluid through eachworking chamber, on a cycle by cycle basis, in phased relationship tocycles of a working chamber volume, to determine the net throughput offluid through the machine. Thus, the fluid working machine operatesaccording to the principles disclosed in EP 0 361 927, EP 0 494 236, andEP 1 537 333, the contents of which are incorporated herein by virtue ofthis reference.

The valve assembly of the present invention is of particular benefit inconnection with fluid working machines of this type, as it can be openedquickly (within a few milliseconds) against a pressure differential,without excessive energy expenditure. Furthermore, as the valve assemblycan be held open by the magnetic circuit arrangement, a substantialvolume of fluid can flow through the valve assembly over a short periodof time without the valve assembly being dragged shut. The valveassembly may be useful as either a low or high pressure valve.

Further modifications and variations may be made within the scope of theinvention herein disclosed.

1. A valve assembly for regulating the supply of fluid from a fluidmanifold to a working chamber of a fluid working machine, the valvecomprising a primary valve, an electromagnet and an armature, theprimary valve comprising a face-seating primary valve member and aprimary valve seat and having an open position in which the primaryvalve member is spaced apart from the primary valve seat and a sealingposition in which the primary valve member is in sealing contact withthe primary valve seat, wherein the armature is slidable along a pathextending between a first position and a second position, and wherein,when the armature is in the first position, the primary valve member isbiased towards the sealing position, and wherein, when the armature isin the second position, the primary valve member is biased towards theopen position, characterised in that the primary valve member comprisesa ferromagnetic member and the valve further comprises a magneticcircuit adapted to direct magnetic flux through the ferromagnetic memberwhen the primary valve is open to thereby hold the primary valve memberin the open position.
 2. A valve assembly according to claim 1, whereinthe magnetic circuit is adapted to direct magnetic flux through theferromagnetic member both when the primary valve member is in the openposition and when the primary valve member is in the sealing position,wherein the magnetic circuit is adapted to direct a higher density ofmagnetic flux through the ferromagnetic member when the primary valvemember is in the open position.
 3. A valve assembly according to claim1, wherein the magnetic circuit comprises first and second magneticcircuit portions which are arranged to conduct magnetic flux from theelectromagnet in parallel, wherein the first magnetic circuit portion isconfigured to conduct magnetic flux through the armature, at least whenthe primary valve is in the sealing position and the armature is in thefirst position, and the second magnetic circuit portion is configured toconduct magnetic flux through the ferromagnetic member, at least whenthe primary valve is in the open position and the armature is in thesecond position.
 4. A valve assembly according to claim 3, wherein thefirst magnetic circuit portion and armature are configured so that theratio of the reluctance of the first magnetic circuit portion to thereluctance of the second magnetic circuit portion is higher when thearmature is in the second position and the primary valve member is inthe open position than when the armature is in the first position andthe primary valve member is in the sealing position, to thereby increasethe magnetic flux directed through the primary valve member when thearmature is in the second position and the primary valve member is inthe open position.
 5. A valve assembly according to claim 4, wherein thefirst magnetic circuit comprises a flux bridge arranged to direct fluxthrough the armature when the armature is in the first position, whereinthe reluctance of the interface between the flux bridge and the armatureis higher when the armature is in the second position than when thearmature is in the first position.
 6. A valve assembly according toclaim 5, wherein the armature overlaps with the flux bridge, wherein thesurface area which overlaps is less when the armature is in the secondposition than the first position to thereby increase the reluctance ofthe interface between the flux bridge and the armature when the armatureis in the second position.
 7. A valve assembly according to claim 5,wherein the armature is adapted to slide along an axis from the firstposition to the second position and the flux bridge comprises aplurality of radially inwardly extending magnetic circuit members.
 8. Avalve assembly according to claim 1, wherein the second magnetic circuitportion is configured to conduct magnetic flux through the ferromagneticmember both when the primary valve member is in the open position andwhen the primary valve member is in the sealing position.
 9. A valveassembly according to claim 1, comprising an elastic member arranged tobias the primary valve member away from the primary valve seat and anelastic member arranged to bias the armature to contact the primaryvalve member such that the resultant forces bias the primary valvetowards the sealing position.
 10. A valve assembly according to claim 1,further comprising a secondary valve coupled to the armature andcomprising a secondary valve member moveable between a sealing positionand an open position, wherein when the armature is in the first positionthe secondary valve is in the sealing position.
 11. A valve assemblyaccording to claim 10, wherein the coupling between the armature and thesecondary valve is configured to enable the armature to move from thefirst position towards the second position without a correspondingmovement of the secondary valve member, but to exert a force through thecoupling between the armature and the secondary valve member to causethe secondary valve member to move and to thereby open the secondaryvalve, while the armature is at a location between the first positionand the second position along the said path.
 12. A valve assemblyaccording to claim 10, wherein the secondary valve member is arranged toprovide a path for fluid to flow between opposite sides of the primaryvalve member in the open position so that, in use, when there is apressure differential across the primary valve member which applies aforce maintaining the primary valve member in sealing contact with theprimary valve seat, opening of the secondary valve member enablespressure to be equilibrated on either side of the primary valve memberto facilitate the opening of the primary valve member.
 13. A valveassembly according to claim 1, wherein the valve assembly is a valveassembly for regulating the supply of fluid from a high-pressuremanifold to a working chamber of a fluid working machine, the valvefurther comprising a secondary valve, the secondary valve comprising asecondary valve member moveable between a sealing position and an openposition in which a path is provided through the secondary valve forfluid to flow between opposite sides of the primary valve member toreduce the pressure difference across the primary valve member, whereinthe armature is coupled to the secondary valve member and the secondposition is closer to the electromagnet than the first position,wherein, in the first position, the secondary valve is biased towardsthe sealing position and, in the second position, the secondary valve isbiased towards the open position, wherein the coupling between thearmature and the secondary valve member is configured to enable thearmature to move from the first position towards the second positionwithout a corresponding movement of the secondary valve member, but toexert a force through the coupling between the armature and thesecondary valve member to cause the secondary valve member to move andto thereby open the secondary valve, while the armature is at a locationbetween the first position and the second position along the said path.14. A fluid working machine comprising a working chamber of cyclicallyvarying volume, a high pressure manifold and a low pressure manifold,and a valve assembly according to claim 1, which regulates the supply offluid from the high pressure manifold or the low pressure manifold tothe working chamber.
 15. A fluid working machine according to claim 14further comprising a controller which is operable to actively controlthe said valve assembly, and optionally one or more other valves, inphased relation to cycles of working chamber volume, to determine thenet displacement of the fluid by the working chamber on a cycle by cyclebasis.